This invention is generally directed to silicon nitride compositions and methods for their preparation and is particularly directed to silicon nitride containing carbon uniformly distributed throughout the silicon-nitrogen matrix.
In structural applications, such as high temperature ceramics, silicon nitride is characterized by low thermal expansion coefficients, moderate thermal conductivities and resistance to thermal shock. However, the difficulty of fabricating silicon nitride into fully dense and suitable shapes has limited its widespread use in such applications.
Two general methods for forming relatively dense-shaped silicon nitride are commonly used. One method, known as "reaction bonding", comprises compacting silicon powder to the desired shape and exposing the silicon compact to molecular nitrogen at about 1400.degree. C. The product is a mixture of .alpha. and .beta. silicon nitride having a porosity of about 25%. Since the original dimensions of the silicon compact remain substantially unchanged during the nitriding, quite complex shapes can be obtained, but the degree of porosity of the product is too high for applications requiring high strength. Another method, known as "hot-pressing", comprises nitriding powdered silicon to form an .alpha. silicon nitride powder and hot-pressing the .alpha. silicon nitride with 1% to 5% by weight of additives such as MgO or Y.sub.2 O.sub.3 at 4000 psi and temperatures between 1700.degree. C. to 1800.degree. C. This method yields fully dense .beta. silicon nitride, but is limited to fairly simple shapes and is very expensive.
To overcome the problems associated with fabricating silicon nitride into useful high temperature materials, e.g., ceramics, a series of compounds known in the art as "SiAlON's" have been developed (K. H. Jack and W. I. Wilson, Nature Phy. Science, Vol. 238, July 1972 and Y. Oyama, Japan J. Appl. Phys., 11, p. 1572, 1972). These compounds result from the substitution of aluminum and oxygen into the .beta. silicon nitride lattice by reacting alumina (Al.sub.2 O.sub.3) with .beta. silicon nitride. The SiAlON unit cell is like that of .beta. silicon nitride and is referred to as .beta.'Si.sub.3 N.sub.4. It contains eight oxygen plus nitrogen atoms and has a range of homogeneity as Si.sub.6.sub.-0.75x Al.sub.0.67x O.sub.x N.sub.8.sub.-x where x is 0 to 6. Unlike unsubstituted silicon nitride, the SiAlON's sinter when fired in an inert atmosphere above 1500.degree. C. and can therefore be processed by the conventional ceramic techniques such as extrusion, pressing and slip-casting to produce pre-fired shapes. SiAlON's are commonly prepared using additives such as AlN and MgO to enhance densification (W. B. Crandell et al., "The Preparation and Evaluation of SiAlON", U.S. contract No. F33615-73-C-4098, Aerospace Research Laboratories, united States Air Force). Although the SiAlON's prepared using additives are sufficiently dense for many applications, they generally exhibit relatively poor properties as a fabricated ceramic in high temperature applications. This is believed to be due to the formation of a magnesium-silicon-aluminum oxynitride crystal phase and/or a silicon-aluminum oxynitride crystal phase in addition to major SiAlON crystal phase, known as .beta.'-Si.sub.3 N.sub.4.
This invention provides for an amorphous silicon nitride which can be directly converted to fully dense uniphase SiAlON having the ease of fabrication of conventionally produced SiAlON and the high strength and thermal shock resistance of fully dense .beta. silicon nitride.